In recent years, means for configuring an electronic circuit by forming a circuit pattern on a plastic board, a ceramic board, or an insulating board coated with plastic or the like, and bonding electronic components such as IC elements, semiconductor chips, resists, and condensers onto the circuit pattern by soldering has been widely used.
In this case, in order to bond a lead terminal of the electronic component to a predetermined portion of the circuit pattern, generally, a thin solder layer is formed in advance on the surface of a conductive circuit electrode on the board, a solder paste or a flux is printed on the thin solder layer, predetermined electronic components are loaded so as to be positioned, and then the thin solder layer or the thin solder layer and the solder paste are caused to reflow to melt and solidify the solder, whereby the lead terminal is bonded by soldering.
In recent years, in order to miniaturize electronic products, the circuit board has been required to have a fine pitch, and for example, fine pitch components such as a QFP (Quad Flat Package) with a pitch of 0.3 mm, a CSP (Chip Size Package), an FC (Flip Chip) with a pitch of 0.15 mm, and an LSI chip having a BGA structure are widely mounted on the circuit board. In order to mount these chips on the circuit board, solder bumps formed in the chip or the like and solder bumps formed in the circuit board are superimposed on each other, and the solder bumps are caused to reflow to each other, whereby both the bumps are melted and bonded. Accordingly, in the circuit board, solder bumps protruding from the circuit board surface need to be formed, and the solder bumps are required to have a fine pattern shape that can respond to the fine pitch.
As means for forming the solder bumps in a circuit board, there is electroplating, non-electrolytic plating, or a method of printing a paste of solder particles and causing the paste to reflow, for example. However, in a method of producing the solder bump by the non-electrolytic plating, it is difficult to make the solder layer thick, and in a method of producing the solder bump by the electroplating, it is difficult to apply electric currents for plating to a complex circuit. Moreover, in a method implemented by printing the paste of solder particles, it is difficult to respond to the fine chip pattern.
As a method of forming a circuit board of a fine pitch, there is a disclosure of a method (see PTL 1). In this method, an adhesive layer is formed by allowing an adhesiveness-imparting compound to react with the surface of a terminal portion of the circuit board, solder particles are attached to the adhesive layer, and then the circuit board is heated to melt the solder particles, whereby solder bumps are formed.
In addition, a technique of covering a circuit electrode portion with a resist to make uniform the height of solder bumps, providing opening portions in the circuit electrode portion, and attaching only one solder particle to each opening portion has been developed (see PTL 2).
Furthermore, a method of using solder particles which are processed into a predetermined shape to improve the adhesiveness of the solder particles has been developed (see PTL 3).